Optical waveguides are physical structures that guide electromagnetic waves in the optical spectrum. These waveguides are typically employed as components in photonic integrated circuits (PICs) or as a transmission medium in local and long haul optical communication systems.
Optical waveguides can be categorized according to their geometry of the optical guiding structure as being slab, channel, ridge, rib or fiber waveguides. Most waveguides used in PICs generally include three planar layers of materials. The bottom layer is a substrate or lower cladding. The middle layer is called the core. The top layer is called a cover or upper cladding. Each layer may have a different dielectric constant. For commercial applications where optical components need to be relatively small and device integration is desirable, photonic-integrated waveguide optical amplifiers are desired.
Generally, there are two classes of waveguide optical amplifiers. This depends on the optical gain material used. In semiconductor optical waveguide amplifiers, waveguides are formed out of semiconductor materials, including InP and InGaAs. In RE-doped waveguide amplifiers, the optical gain is provided by rare-earth ions, such as Erbium (Er), Yitterbium (Yb), or Thulium (Tm), embedded in host waveguides, typically made of a dielectric insulator. RE-doped waveguide amplifiers are advantageous for providing optical gains for signals consisting of multiple wavelengths or amplifying optical pulses having optical energies exceeding pJ.
Planar PIC RE-doped waveguide amplifiers are not as widely deployed in the field. By contrast, RE-doped fiber optical amplifiers are used in optical communication as the standard optical gain elements. Prior-art, planar RE-doped waveguide amplifiers include an all-glass geometry, whereby the substrate and cover include passive (i.e., undoped with RE) silica glass and the core includes glassy host materials doped with RE ions. Fabrication of such doped waveguide cores can be complex, leading to limited market penetration. In addition, the potential for reduction of the prior-art RE-doped PIC size in comparison to equivalent fiber devices is not significant owing to the small index contrast between the undoped and doped materials.
What is desired in the art is RE-doped waveguide structure that can support tighter waveguide bend radii. What is also desired is a more compact structure. It is also desired that such structure have small background optical loss so that optical gain can be obtained in power efficient manner.
What is also desired in the art is a RE-doped PIC waveguide amplifier that can be fabricated using a process that is more conducive to mass-scale production. Prior-art RE-doped waveguide processing is not CMOS process compatible.